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When austenite is deformed within the austenite phase field, that is, above the equilibrium transformation temperature Ae3, it partially transforms dynamically into ferrite. Here, rough rolling simulations are conducted at 1100 °C on an X70 steel to study the influence of grain size and residual strain on this type of transformation. Three simulated roughing passes are applied with pass strains of 0.4 at a strain rate of 1.0 s−1. The resulting stress–strain curves are recorded and analyzed. The volume fractions of ferrite and martensite (prior austenite) are determined on quenched samples by means of metallographic techniques. It is observed that dynamic transformation took place during the first roughing pass and that the volume fraction of transformed ferrite increases with the cumulative strain. Both the critical strain to the onset of dynamic transformation as well as the grain size decrease with pass number. [ABSTRACT FROM AUTHOR]

The dynamic transformation (DT) of austenite to ferrite and the retransformation of the dynamically transformed ferrite back into the more stable austenite were studied at temperatures above the Ae3. For this purpose, compression tests were carried out on a C-Mn and a Nb microalloyed steel over the temperature range 870-960 °C. The samples were deformed to a strain of 0.75 at a strain rate of 1 s−1 and held isothermally for times of 1-1000 s to permit retransformation. The samples were water quenched after testing and the volume fractions of retransformed ferrite were measured to allow for the construction of TTRT (time-temperature-reverse transformation) curves. The present type of diagram can be combined with strain-temperature-transformation (STT) curves to model the occurrence of dynamic transformation and retransformation during thermomechanical processing. [ABSTRACT FROM AUTHOR]

The dynamic transformation (DT) of austenite to ferrite and the retransformation of the dynamically transformed ferrite back into the more stable austenite were studied at temperatures above the Ae3. For this purpose, compression tests were carried out on a C-Mn and a Nb microalloyed steel over the temperature range 870-960 °C. The samples were deformed to a strain of 0.75 at a strain rate of 1 s−1 and held isothermally for times of 1-1000 s to permit retransformation. The samples were water quenched after testing and the volume fractions of retransformed ferrite were measured to allow for the construction of TTRT (time-temperature-reverse transformation) curves. The present type of diagram can be combined with strain-temperature-transformation (STT) curves to model the occurrence of dynamic transformation and retransformation during thermomechanical processing. [ABSTRACT FROM AUTHOR]

A five-pass rolling schedule is simulated by means of torsion testing. The first and last pass temperatures are 920 and 860 °C with 15° of cooling between passes. All of the rolling is carried out above the Ae3 temperature of 845 °C that applies to this steel. Interpass times of 10, 20, and 30 s are employed, which correspond to cooling rates of 1.5, 0.75, and 0.5 °C s−1, respectively. Samples were quenched before and after the first, third, and fifth passes in order to determine the amount of dynamic ferrite produced in a given pass. The data also allow for estimation of the amounts of ferrite that retransform to austenite during the time between passes. The volume fractions of ferrite and martensite (prior austenite) were determined by optical metallography. The amount of dynamic ferrite formed and retained increased with pass number. The volume fraction of ferrite that retransformed increased with pass number, averaging about 4, 6, and 9% for the 10, 20, and 30 s interpass times, respectively. The simulations indicate that ferrite is unavoidably produced during plate rolling and that the microstructure present at the initiation of accelerated cooling does not consist solely of austenite. [ABSTRACT FROM AUTHOR]

It has been previously demonstrated that austenite may undergo partial dynamic transformation (DT) during the plate rolling process. Austenite dynamically transforms into unstable ferrite during hot deformation even at very high temperatures. In this work, the plate rolling simulations, with emphasis on Steckel mill operations, through torsion testing under isothermal conditions were performed on an X70 steel. Four different roughing schedules were tested followed by five finishing passes with pass strains of 0.3 applied at 900 °C. The roughing schedules had zero, one, two and three roughing passes at a temperature of 1100 °C, strain of 0.4 and strain rate of 1 s−1. The stress–strain curves as well as the mean flow stress (MFS) behaviors indicated that both dynamic transformation (DT) and dynamic recrystallization (DRX) occurred during straining. The critical strains for the onset of DT and DRX were determined by means of the double differentiation method and the critical strain values decreased with the number of roughing and finishing strains from the first going to the last pass. It was observed that the volume fraction of the dynamically formed ferrite increased sharply during the finishing stage as the number of previous roughing passes increased, which can be attributed to higher strain accumulation. The results presented here indicate that improved models are needed to control the microstructure of the material during subsequent cooling. [ABSTRACT FROM AUTHOR]